Image forming apparatuses, such as laser printers, form a toner image onto a recording medium by transferring the toner image from a photosensitive drum. In such image forming apparatuses, an electrostatic latent image formed on a photosensitive drum is developed by using toner (a developing agent) to form a toner image on a surface of the photosensitive drum. Then, by using a transfer roller to which a transfer bias is applied and that rotates with pressing the photosensitive drum, the toner image is electrically transferred to a recording medium from the photosensitive drum while the recording medium is pinched at a pressing portion (a nip portion) between the photosensitive drum and the transfer roller.
For example, as shown in FIG. 6, such a transfer roller commonly includes a roller portion 211 that presses a photosensitive drum 201 and a rotating shaft 212 that protrude from both ends of the roller portion 211 in an axial direction of the transfer roller.
In order to press the roller portion 211 of the transfer roller against the photosensitive drum 201, an urging device (e.g. a compression spring) that urges the rotating shaft 212 toward the photosensitive drum 201 is provided. In a transfer device of FIG. 6, however, the rotating shaft 212 of the transfer roller is urged at its both end portions, so that the rotating shaft 212 and the roller portion 211 are warped such that middle portions thereof move away from the photosensitive drum 201 as the both end portions thereof move closer to the photosensitive drum 201 (in the drawing, the warp of the rotating shaft 212 is exaggerated for the sake of clarity). As a result, a pressing force of the transfer roller that acts on the photosensitive drum 201 may vary between the end portions and the middle portion of the roller portion 211 of the transfer roller. If the pressing force of the transfer roller varies as described above, a nip width between the transfer roller and the photosensitive drum 201 may become non-uniform, resulting in a transfer current varying by portion. If the transfer current is not maintained within a substantially constant range, image quality may be degraded.
In order to resolve the above problem, for example, Japanese Laid-Open Patent Publication No. 10-198197 discloses a structure such that cylindrical rollers are provided to a rotating shaft of a transfer roller in order to regulate a pressing force of a roller portion of the transfer roller that acts on a photosensitive drum.
However, the rollers are generally made of a material having high stiffness, so that the pressing force of the roller portion of the transfer roller that acts on the photosensitive drum (hereinafter, referred to as a roller pressing force) significantly varies if variations occur in a diameter of the rollers. More specifically, when the rollers have a larger diameter than an optimum diameter, the roller pressing force becomes weak as compared with the roller pressing force when the rollers have the optimum diameter. When the rollers have a smaller diameter than the optimum diameter, the roller pressing force becomes strong. That is, in order to maintain an optimum roller pressing force, the rollers need to be manufactured with very high precision. If there is a dimension error (e.g., difference in a diameter, or deviation of the center) between the right and left rollers, the nip width between the transfer roller and the photosensitive drum considerably varies at the right and left portions. Therefore, a transfer current becomes non-uniform and thus a defect may occur in an image transfer. In addition, because the rollers are made of a material having high stiffness, the rollers may cause damage to a member with which the rollers make contact (e.g., a photosensitive drum, an intermediate transfer belt, or a sheet transfer belt).
This effect may also be seen or further compounded if there is play in a driving system that also supports the rotating shaft of the transfer roller.